Diamond is an essential material for the manufacture of semiconductors devices, infrared optical windows, heat dissipation and other fields. The applications have put forward a practical demand that the diamond substrates have an ultra-smooth surface and are free of structural damage. However, precision polishing of diamond is challenging due to the extreme hardness and chemical inertness. The focus of this study is to prepare nanoscale smooth polycrystalline diamond films with roughness Ra down to 0.1 nm using chemical and mechanical synergies. After processing, the surface roughness and quality of CVD diamond film were characterized, and a material removal mechanism was proposed. The graphitization process is accelerated when diamond contacts the transition metal Fe in the mechanical polishing process. In chemical mechanical polishing process, the polished surface of diamond C is transformed into deformed diamond C* under friction. A relatively soft polishing pad is used to increase the contact area with the diamond, which is transformed into a softer, easy-to-remove structure under the action of potassium permanganate oxidant, and then the reaction layer is removed again by mechanical friction.